Fatty acids are versatile building blocks used in many parts of the chemical industry, in applications ranging from lubricants, polymers, and solvents to cosmetics and health care. Fatty acids are generally obtained by the hydrolysis of triglycerides of vegetable or animal origin. Naturally occurring triglycerides are esters of glycerol and generally straight chain, even numbered carboxylic acids, ranging from 10 to 24 carbon atoms. Most common are fatty acids having 12, 14, 16 or 18 carbon atoms. The fatty acids can be either saturated or contain one or more unsaturated carbon bonds.
Straight chain saturated fatty acids having 10 or more carbon atoms are solid at room temperature, which makes them difficult to process in a number of applications. Unsaturated long chain fatty acids, e.g. oleic acid, are liquid at room temperature, and so are easy to process, but are unstable because of the existence of one or more double bonds. Branched fatty acids can mimic the properties of the straight chain unsaturated fatty acids in many respects. However, they do not have the disadvantage of being unstable. Thus, branched fatty acids are, for many applications, more desirable than straight chain fatty acids. The branched fatty acids have alkyl side groups which are generally short, e.g. methyl, ethyl or propyl, and can be attached to the carbon chain backbone at any position.
Commercially available branched fatty acids such as isostearic acid, are obtained as a by-product of the catalytic or thermal dimerisation of unsaturated straight chain fatty acids. Isostearic acid is produced by heating oleic acid in the presence of catalyst, generally clay, to produce dimer, trimer and higher oligomer acids. But instead of polymerising, a portion of the oleic acid rearranges to give a branched, monomeric fatty acid which can be isolated by distillation and hydrogenated. This saturated branched monomeric fatty acid is a mixture of various linear and mainly branched, both mono and poly branched, saturated acids which is known as isostearic acid.
Isostearic acid exhibits better stability to oxidation than oleic acid, and is a very useful product which is sold into a wide range of application areas such as lubricant esters, and cosmetic applications. Isostearic acid is also used to make isostearyl alcohol.
The dimerisation process only produces about 20 to 40% by weight of isostearic acid, and thus there is a need for a more efficient process. A further disadvantage, which increases the cost of the process, is that the clay catalyst cannot be reused.
EP-0683150 describes an alternative process for producing branched fatty acids by using a zeolite catalyst which has a linear pore structure. This process has a much higher selectivity towards monomeric than dimeric or oligomeric products.
Isostearic acid can be further purified, e.g. as described in EP-0415697, using urea clathration. But this additional processing stage increases the overall cost of the process.
Thus, there is a need for an improved process of producing monobranched fatty acids, particularly highly monobranched fatty acids.